Steel material having excellent sulfide stress corrosion cracking resistance and method of manufacturing same

ABSTRACT

The present disclosure relates to a thick steel material that can be appropriately used as a line pipe, a sour-resistant material and, more particularly, to a high-strength steel material having excellent sulfide stress corrosion cracking resistance and excellent resistance against propagation of sulfide stress corrosion cracking, and a method of manufacturing the steel material.

TECHNICAL FIELD

The present disclosure relates to a thick steel material that issuitable for a line pipe, a sour-resistant material and, moreparticularly, to a high-strength steel material having excellent sulfidestress corrosion cracking resistance, and a method of manufacturing thesteel material.

BACKGROUND ART

Recently, the demand for an upper limit of surface hardness of line pipesteel materials is increasing. When the surface hardness of a line pipeis high, non-uniformity of roundness is caused when a pipe is machined,and cracks are formed due to high-hardness structures of the pipesurface when the pipe is machined and a deficit of toughness is causedin a use environment. Further, there is a high possibility that thehigh-hardness structures of the surface may cause brittle cracking dueto hydrogen when the material is used in a sour environment with a lotof sulfides, so there is a high possibility of a significant accident.

There is an instance in which, in 2013, sulfide stress cracking (SSC)occurred in the high-hardness portions of a pipe surface within 2 weeksof a large-scale petroleum/natural gas exploitation project beingstarted at the Caspian Sea, so pipelines at 200 km below the sea werereplaced with clad pipes. In this case, as the result of analysis,formation of hard spots that are high-hardness structures of the pipesurface is inferred as the reason of SSC.

The length is regulated at 2 inches or more and hardness is regulated atHv 345 or more for hard spots under API standards, and DNV standardsregulate the same sizes as API standards, but regulate the upper limitof hardness at Hv 250.

Meanwhile, steel materials for line pipe are manufactured generally byreheating, hot-rolling, and then accelerated-cooling a steel slab, andit is determined that hard spots (portions at which high-hardnessstructures are formed) are generated due to non-uniform rapid cooling ofa surface portion in the accelerated cooling.

In a steel plate manufactured by common water cooling, the cooling rateis higher at the surface portion than the center portion because wateris sprayed to the surface of the steel plate and hardness in the surfaceportion is higher than the center portion due to the cooling ratedifference.

A method of attenuating a water-cooling process may be considered as amethod for suppressing formation of high-hardness structures at thesurface portion of a steel material. However, reducing surface hardnessby attenuating water cooling is accompanied by strength reduction, whichcauses a problem that more alloy elements should be added, etc. Further,such an increase of alloy elements is also a factor that increasessurface hardness.

RELATED ART DOCUMENT Patent Document

(Patent Document 1) Korean Patent Application Publication No.1998-028324

DISCLOSURE Technical Problem

An aspect of the present disclosure is to provide a high-strength steelmaterial having excellent sulfide stress corrosion cracking resistanceby effectively reducing hardness at a surface portion in comparison to athick-plate water-cooled material (TMCP) by optimizing alloy compositionand manufacturing conditions, and a method of manufacturing thehigh-strength steel material.

In more detail, an aspect of the present disclosure is to provide ahigh-strength steel material having yield strength of 450 MPa or moreand having excellent sulfide stress corrosion cracking resistance in ahigh-pressure H₂S environment exceeding partial pressure of 1 bar, and amethod of manufacturing the high-strength steel material.

Further, an aspect of the present disclosure is to secure also apropagation resistance against sulfide stress corrosion cracking byincreasing sulfide stress corrosion cracking resistance by effectivelycontrolling hardness of a surface portion at a low level throughoptimization of alloy composition and manufacturing conditions, and byminimizing the content of chrome (Cr) that accelerates propagation ofsulfide stress corrosion cracking in a high-pressure H₂S environment.

The objectives of the present disclosure are not limited to thatdescribed above. Those skilled in the art may understand additionalobjectives of the present disclosure without difficulty from the generalcontents in the specification.

Technical Solution

An aspect of the present disclosure provides a steel material thatincludes, by weight %, carbon (C): 0.02˜0.06%, silicon (Si): 0.1˜0.5%,manganese (Mn): 0.8˜1.8%, chrome (Cr): less than 0.05%, phosphorous (P):0.03% or less, sulfur (S): 0.003% or less, aluminum (Al): 0.06% or less,nitrogen (N): 0.01% or less, niobium (Nb): 0.005˜0.08%, titanium (Ti):0.005˜0.05%, calcium (Ca): 0.0005˜0.005%; one or more of nickel (Ni)0.05˜0.3%, molybdenum (Mo) 0.02˜0.2%, and vanadium (V): 0.005˜0.1%, anda balance of Fe and unavoidable impurities, in which the Ca and the Ssatisfy the following Equation 1, and the steel material has amicrostructure of a surface portion composed of ferrite or a complexstructure of ferrite and pearlite, and a microstructure of the centerportion is composed of acicular ferrite,

0.5≤Ca/S≤5.0  [Equation 1]

where each element represents the content of each element by weight %.

Another aspect of the present disclosure provides a method ofmanufacturing a steel plate that includes: heating a steel slabsatisfying the alloy composition described and Equation 1 at atemperature range of 1100˜1300° C. for 2 hours or more; manufacturing ahot-rolled plate by hot-rolling the heated steel slab; and cooling thehot-rolled plate after hot rolling, in which the cooling includesprimary cooling, air cooling, and secondary cooling, and the primarycooling is performed at a cooling rate of 5˜40° C./s such that atemperature of a surface portion of the hot-rolled plate becomes Ar1−50°C.−Ar3−50° C. and the secondary cooling is performed at a cooling rateof 50˜500° C./s such that the temperature of the surface portion of thehot-rolled plate becomes 300˜600° C.

Advantageous Effects

According to the present disclosure, when a thick steel material havinga predetermined thickness, hardness at a surface portion is effectivelyreduced, so it is possible to provide a high-strength steel materialhaving excellent resistance against sulfide stress corrosion cracking.

Further, according to the present disclosure, it is possible to providea high-strength steel material having excellent resistance againstsulfide stress corrosion cracking and excellent resistance againstpropagation of sulfide stress corrosion cracking too.

This steel material of the present disclosure can be advantageouslyapplied as not only the material of pipes such as a line pipe, but asour-resistant material, and particularly, it is possible to provide ahigh-strength steel material having an excellent sulfide stresscorrosion cracking characteristic even in a high-pressure H₂Senvironment over partial pressure of 1 bar.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows microstrucures and hardness of surface portions ofinvention steel and comparative steel in an experimental example of thepresent disclosure

BEST MODE

At present, Thermo-Mechanical Control Process (TMCP) materials that aresupplied to the market of thick plate materials and hot rolling has acharacteristic that hardness is higher at the surface portion than thecenter portion due to an avoidable phenomenon that is generated incooling after hot rolling (a phenomenon that the cooling rate becomeshigher at the surface portion than the center portion).

Accordingly, as the strength of a material is increased, hardness isconsiderably increased at the surface portion than the center portion,and such an increase in hardness at a surface portion is a factor thatcauses cracking or deteriorates low-temperature toughness in machining.Further, there is a problem that the increase is the onset point ofhydrogen embrittlement when a steel material is applied to a sourenvironment. In spite of such problem of the related art, at present, asteel material having excellent sulfide stress corrosion crackingresistance under a high-pressure H₂S environment is not provided.

Accordingly, the inventors, as the result of recognizing and minutelyexamining the problem of the related art, have found out and achieved asteel material that can effectively suppress sulfide stress corrosioncracking resistance due to hard spots and does not easily propagatecracks even if cracks are generated at a surface portion due to hardspots.

In detail, the inventors, as an aspect of the present disclosure, haveintended to provide a steel material securing resistance againstcracking and propagation resistance against cracking and having highstrength by effectively decreasing hardness of a surface portion in athick steel plate having a predetermined thickness or more.

The inventors has conceived a new cooling control technique rather thanthe common cooling method of the related art, whereby the inventors haveconceived a technique that can attenuate hardness of a surface portionseparating phase transformation at a surface portion and a centerportion.

That is, the inventors have develop a technique that can reducehardenability of a surface portion by promoting decarburization of thesurface portion in the process of heating and rolling, and can formferrite at the surface portion. Further, the inventors of the presentdisclosure intend to provide a technique of manufacturing a steel platehaving excellent sulfide stress corrosion cracking resistance even undera high-pressure H₂S environment by optimizing the components of steeland conditions such as manufacturing process (heating, hot rolling,cooling, etc.) because they have found out that when Cr is added as analloy elements in a steel material propagation resistance againstsulfide stress corrosion cracking resistance is deteriorated.

Hereafter, the component system of a steel material according to thepresent disclosure is described first.

A steel material according to an embodiment of the present disclosuremay include, in percent by weight, carbon (C): 0.02˜0.06%, silicon (Si):0.1˜0.5%, manganese (Mn) 0.8˜1.8%, chrome (Cr): less than 0.05%,phosphorous (P): 0.03% or less, sulfur (S): 0.003% or less, aluminum(Al) 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb)0.005˜0.08%, titanium (Ti): 0.005˜0.05%, calcium (Ca): 0.0005˜0.005%;one or more of nickel (Ni): 0.05˜0.3%, molybdenum (Mo): 0.02˜0.2%, andvanadium (V): 0.005˜0.1%, and a balance of Fe and unavoidableimpurities.

Hereafter, the reason of limiting the alloy composition of the steelmaterial provided in the present disclosure, as described above, isdescribed in detail.

Meanwhile, unless specifically stated in the present disclosure, thecontent of each element is based on weight and the ratio of structuresis based on an area.

Carbon (C): 0.02˜0.06%

Carbon is an element having the largest influence on the properties ofsteel. When the content of C is less than 0.02%, there is a problem thatan excessive component control cost is generated in the steelmanufacturing process and welding heat-influenced portions areexcessively softened. However, when the content exceeds 0.06%, hydrogeninduced cracking resistance of a steel plate is decreased and weldabiitymay be deteriorated. Accordingly, in the present disclosure, C may beincluded at 0.02˜0.06%, and more preferably, 0.03˜0.05%.

Silicon (Si): 0.1˜0.5%

Silicon (Si) is an element that not only is used as a deoxidizer in asteel manufacturing process, but serves to increase strength of steel.When the content of Si exceeds 0.5%, low-temperature toughness of amaterial, weldability, and scale separability in rolling aredeteriorated. Meanwhile, the manufacturing cost is increased to reducethe content of Si less than 0.1%, so the content of Si may be limited at0.1˜0.5%, and more preferably, 0.2˜0.4%.

Manganese (Mn): 0.8.0˜1.8%

Manganese (Mn), which is an element that improves hardenability of steelwithout deteriorating low-temperature toughness, may be included at 0.8%or more. However, when the content exceeds 1.8%, centerline segregationoccurs, so there is a problem that low-temperature toughness isdeteriorated, hardenability of steel is increased, and weldability isdeteriorated. Further, centerline segregation of Mn is a factor thatcauses hydrogen induced cracking. Accordingly, Mn may be included at0.8˜1.8% in the present disclosure. Alternatively, in terms ofcenterline segregation, Mn may be included preferably at 0.8˜1.6%, andmore preferably, 1˜1.4%.

Chrome (Cr): Less than 0.05%

Chrome (Cr) is solidified in austenite when a slab is reheated, therebycontributing to increasing hardenability of a steel material andsecuring strength of a steel plate. However, the inventors have foundout that when Cr is added at 0.05% or more, propagation of sulfidestress corrosion cracking may be promoted. That is, the content of Cr islimited less than 0.05% in a steel material, thereby achieving an effectthat resistance against propagation of sulfide stress corrosioncracking. Meanwhile, the steel material according to an aspect of thepresent disclosure may include Cr more than 0% and less than 0.05%, morepreferably, 0.04% of less, and the most preferably, 0.02% or less.However, since Cr may not be added when strength can be secured, thelower limit of the content of Cr may be 0%, and preferably, 0.0005%.

Phosphorous: 0.03% or Less

Phosphorous (P) is an element that is unavoidably added in steel, andwhen the content exceeds 0.03%, there is a problem that not onlyweldability is remarkably decreased, but low-temperature toughness isreduced. Accordingly, it is required to limit the content of P at 0.03%or less, and, in terms of securing low-temperature toughness, morepreferably, P may be included at 0.01% or less. However, 0% may beexcluded as the lower limit of the content of Cr in consideration ofload in the steel manufacturing process, and more preferably, the lowerlimit of the content of Cr may be 0.0001%.

Sulfur (S): 0.003% or Less

Sulfur (S) is an element that is unavoidably added in steel, when thecontent exceeds 0.003%, there is a problem that ductility,low-toughness, and weldability of steel are reduced. Accordingly, thecontent of S needs to be limited at 0.003% or less. Meanwhile, Sproduces a MnS inclusion by bonding with Mn in steel, and in this case,the hydrogen induced cracking resistance of steel is deteriorated, so,more preferably, S may be included in 0.002% or less. However, 0% may beexcluded as the lower limit of the content of S in consideration of loadin the steel manufacturing process, and more preferably, the lower limitof the content of S may be 0.0001%.

Aluminum (Al): 0.06% or Less (Excluding 0%)

Aluminum (Al) usually functions as a deoxidizer that removes oxygen byreacting with oxygen (O) existing in molten steel. Accordingly, Al maybe added such that it has a sufficient decarburization ability in steel.However, when the content exceeds 0.06%, a large amount of oxide-basedinclusion is produced and deteriorates low-temperature toughness,hydrogen induced cracking resistance, and sulfide stress corrosioncracking resistance, which is not preferable. Accordingly, Al may beincluded at 0.06% or less, and more preferably, 0.04% or less. However,0% may be excluded as the lower limit of the content of S inconsideration of that Al is necessarily included as a deoxidizer, andmore preferably, the lower limit of the content of Al may be 0.005%.

Nitrogen (N); 0.01% or Less (Excluding 0%)

Nitrogen (N) is difficult to be industrially completely removed fromsteel, so the upper limit thereof is 0.01% that is an allowable range ina manufacturing process. Meanwhile, since N produces nitrides byreacting with Al, Ti, Nb, V, etc. in steel, N suppresses growth ofaustenite grains, which has a good influence on improvement of toughnessand strength of a material. However, when N is excessively added over0.01%, N exists in a solidified state, which has a bad influence onlow-temperature toughness. Accordingly, N may be included at 0.01% orless, and more preferably, 0.009% or less. However, 0% may be excludedas the lower limit of the content of N in consideration of load in thesteel manufacturing process, and more preferably, the lower limit of thecontent of N may be 0.0005%.

Niobium (Nb): 0.005˜0.08%

Niobium (Nb) is an element that solidifies when a slab is heated,thereby suppressing growth of austenite grains and effectively improvingstrength of steel through precipitation. Further, Nb is precipitated asa carbide by bonding with C in steel, thereby serving to minimize anincrease of a yield ratio and improving strength of steel. When thecontent of Nb is less than 0.005%, the above effect cannot besufficiently obtained, but when the content exceeds 0.08%, there is aproblem that not only austenite grains are unnecessarily excessivelymicronized, but low-temperature toughness and hydrogen induced crackingresistance are deteriorated due to production of coarse precipitates.Accordingly, Nb may be included within 0.005˜0.08% in the presentdisclosure. Meanwhile, the lower limit of the content of Nb may be morepreferably 0.02% and the upper limit of the content of Nb may be 0.05%.

Titanium (Ti): 0.005˜0.05%

Titanium (Ti) is precipitated as TiN by bonding with N when a slab isheated, which is effective in suppression of growth of austenite grains.When Ti is added less than 0.005%, austenite grains are coarsened, solow-temperature toughness is reduced. However, when the content exceeds0.05%, coarse Ti-based precipitates are produced, so low-temperaturetoughness and hydrogen induced cracking resistance are reduced.Accordingly, Ti may be included within 0.005˜0.05% in the presentdisclosure. Meanwhile, the lower limit of the content of Ti may be morepreferably 0.006% and the upper limit of the content of Ti may bepreferably 0.03% in terms of securing low-temperature toughness.

Calcium (Ca): 0.0005˜0.005%

Calcium (Ca) produces CaS by bonding with S in a steel manufacturingprocess, thereby suppressing segregation of MnS that causes hydrogeninduced cracking. It is required to add Ca at 0.005% or more in order tosufficiently achieve the effect of suppressing segregation of MnS, butwhen the content exceeds 0.005%, not only CaS, but CaO inclusions areproduced, so hydrogen induced cracking is caused by the inclusions.Accordingly, in the present disclosure, Ca may be included at0.0005˜0.005%, and more preferably, 0.001˜0.003% in terms of securinghydrogen induced cracking resistance.

The steel material according to the present disclosure contains Ca andS, as described above, in which it is preferable that the compositionratio of Ca and S (([Ca]/[S]) satisfies the following Equation 1.

0.5≤[Ca]/[S]≤5.0  [Equation 1]

where [Ca] is the average content of Ca in a steel material by weight %and [S] is the average content of S in a steel material by weight %).That is, the composition ratio of Ca and S is a representative index forcore segregation of MnS and production of coarse inclusions, and whenthe [Ca]/[S] value is less than 0.5, MnS is produced at the centerportion in the thickness direction of a steel material, which may causea problem of reduction of hydrogen induced cracking resistance. On thecontrary, when the Ca]/[S] value exceeds 5.0, Ca-based coarse inclusionsare produced, which deteriorates hydrogen induced cracking resistance.Accordingly, it is preferable that the composition ratio o Ca and S([Ca]/[S]) satisfies Equation 1, and in order to further improve theabove effect, more preferably, the [Ca]/[S] value may be within therange of 1.4˜3.2.

Meanwhile, the steel material of the present disclosure may furtherinclude elements that can further improve properties other than thealloy composition described above, and in detail, may further includeone or more of Nickel (Ni): 0.05˜0.3%, Molybdenum (Mo): 0.02˜0.2%, andVanadium (V): 0.005˜0.1%. In this case, the steel material has only toinclude one or more of Ni, Mo, and V within the range of being able toachieve the objectives of the present disclosure, and all of Ni, Mo, andV are not necessarily included in the present disclosure.

Nickel (Ni): 0.05˜0.3%

Nickel (Ni) is an element that has an effect in improvement of strengthof steel without deterioration of low-temperature toughness. Ni may beadded at 0.05% or more to achieve the effect of increasing strengthwithout deteriorating low-temperature toughness, but Ni is an expensiveelement and the manufacturing process is considerably increased when thecontent of Ni exceeds 0.3%. Accordingly, Ni may be included at 0.05˜0.3%when Ni is added in the present disclosure. Meanwhile, the lower limitof the content of Ni may be preferably 0.08%, and more preferably, 0.1%.Alternatively, the upper limit of the content of Ni may be preferably0.28%, and more preferably, 0.21%.

Molybdenum (Mo): 0.02˜0.2%

Molybdenum (Mo), similar to Cr, improves hardenability of a steelmaterial and increases strength. Mo may be added at 0.02% or more toachieve the effect of improving hardenability described above, but whenthe content exceeds 0.2%, there is a problem that a structure that isvulnerable to low-temperature toughness such as upper bainite isproduced, and hydrogen induced cracking resistance and sulfide stresscorrosion cracking resistance are deteriorated. Accordingly, Mo may beincluded at 0.02˜0.2% when Mo is added in the present disclosure.Meanwhile, the lower limit of the content of Mo may be more preferably0.05% and the upper limit of the content of Mo may be 0.15%.

Vanadium (V): 0.005˜0.1%

Vanadium (V) is an element that improves strength by increasinghardenability of a steel material and may be added at 0.005% or more toachieve this effect. However, when the content exceeds 0.1%,hardenability of steel excessively increases, so structures that arevulnerable to low-temperature toughness are formed and hydrogen inducedcracking resistance is reduced. Accordingly, V may be included at0.005˜0.1% when V is added in the present disclosure. Meanwhile, thelower limit of the content of V may be more preferably 0.005% and theupper limit of the content of V may be more preferably 0.05%.

The balance is F in the present disclosure. However, since unintendedimpurities may be unavoidably mixed from a raw material or a surroundingenvironment in a common manufacturing process, it cannot be excluded.Since anyone of those skilled in a common manufacturing process can knowsuch impurities, they are not all specifically stated therein.

The steel material having the above alloy composition according to anaspect of the present disclosure is characterized in that themicrostructure of the surface portion is composed of ferrite or acomplex structure of ferrite and pearlite, whereby Vickers hardness ofthe surface portion may be controlled at 200 Hv or less.

Meanwhile, the surface portion is the portion from the surface to apoint at 1000 μm in the thickness direction, which may be applied toboth sides of a steel material. Further, the center portion is the otherregion except for the surface portion.

Further, in the present disclosure, the hardness of the surface portionis a maximum hardness value measured under 1 kgf load using Vickershardness from the surface to a point at 1000 μm in the thicknessdirection. In general, hardness may be measured around 5 times at eachposition.

That is, in the steel material according to the present disclosure, themicrostructure is composed of ferrite or a complex structure of ferriteand pearlite and the microstructure of the center portion is composed ofacicular ferrite, so it is possible to form a soft microstructure at thesurface portion in comparison to the center portion, whereby it ispossible to provide a steel material of which the hardness in thesurface portion is lower than those of existing TMCP steel materials.

In detail, the same or high strength is secured in the steel materialaccording to an aspect of the present disclosure in comparison toexisting TMCP steel materials, so the steel material has yield strengthof 450 MPa or more, the hardness of the surface portion is remarkablyreduced, and the content of Cr is minimized, whereby it is possible toeffectively suppress formation and propagation of sulfide stresscorrosion cracks.

Meanwhile, a method of manufacturing the steel material according to thepresent disclosure described above is described in detail hereafter.

The steel material of the present disclosure may be manufactured througha process of [slab heating—hot rolling—cooling] and each of the processconditions are described in detail hereafter.

[Slab Heating]

A steel slab that satisfies the alloy composition and componentrelationship proposed in the present disclosure may be prepared and thenheated, which may be performed at 1100˜1300° C. for 2 hours.

When the heating temperature exceeds 1300° C., not only a scale defectincreases, but austenite grains are coarsened, so hardness of the steelmay be increased. Further, the fracture of structures that arevulnerable to low-temperature toughness such as upper bainite isincreased at the center portion, so there is a problem that hydrogeninduced cracking resistance and low-temperature toughness resistance aredeteriorated.

However, when the temperature is less than 1100° C. or the heating timeis less than 2 hours, decarburization at the surface portion isinsufficient, which not only adversely influences formation of ferriteat the surface portion, but decrease the re-solidification ratio ofalloy elements. Accordingly, in the present disclosure, the steel slabdescribed above may also be heated for 2 hours or more in thetemperature range of 1100˜1300° C., and more preferably, for 3.0 hoursor more in the temperature range of 1145˜1250° C. Meanwhile, the upperlimit of the slab heating time is not specifically fixed, and generally,since the more the heating time, the higher the component uniformity,the heating time may be 50 hours or less, 20 hours or less, or 6 hoursor less.

Hot Rolling

A hot-rolled plate may be manufactured by hot-rolling the heated steelslab. In this case, hot rolling may be performed at an accumulatedreduction ratio of 50% or more within the temperature range of Ar3+80°C.˜Ar3+200° C., and resting may be maintained for 30 seconds or more(air cooling) after hot rolling.

When the temperature is higher than Ar3+200° C. in hot rolling,structures that are vulnerable to low-temperature toughness such asupper bainite are formed due to an increase of hardenability by growthof grains, so that hydrogen induced cracking characteristics andlow-temperature toughness may be deteriorated.

However, when the temperature is lower than Ar3+80° C., the temperatureat which following cooling is started is excessively low, so thefracture of air-cooled ferrite excessively increases and strength may bedecreased. Further, decarburization at the surface portion issuppressed, so it is difficult to contribute to form ferrite at thecenter portion in the following process. Accordingly, it is preferablein the present disclosure that the finishing rolling temperature of hotrolling is Ar3+80° C.˜Ar3+200° C.

When the accumulated reduction ratio is less than 50% in hot rolling inthe temperature range described above, recrystallization by rolling evento the center portion of the steel material does not occur, so there isa problem that grains are coarsened in the center portion andlow-temperature toughness is deteriorated. Accordingly, it is preferablein the present disclosure that the accumulated reduction ratio is 50% ormore in hot rolling.

Meanwhile, the maintaining time is less than 30 seconds after hotrolling, the time for decarburization at the surface portion isinsufficiency, so it is difficult to contribute to forming ferrite atthe surface portion in the following process. Accordingly, it ispreferable in the present disclosure that the maintaining time afterfinishing hot rolling is 30 seconds or more. The upper limit of themaintaining time after finishing hot rolling is not specifically fixed,but may be preferably 30 minutes or less, 10 minutes or less, or 5minutes or less. Further, since such maintaining time is provided,cooling start temperature to be described below can be secured from aircooling.

[Cooling]

The hot-rolled plate manufactured through such hot rolling can becooled, and particularly, it would be technically meaningful to providean optimal cooling process that can obtain a steel material of whichhardness in the surface portion is effectively reduced in the presentdisclosure.

In detail, the cooling includes primary cooing; air cooling, andsecondary cooling, and each of the process conditions are described inmore detail hereafter.

In this case, the primary cooling and the secondary cooling may beperformed by applying a specific cooling means, and for example, watercooling may be performed.

Primary Cooling

In the present disclosure, primary cooling may be performed after hotrolling—maintaining time over 30 seconds described above is maintained.In detail, it is preferable to start primary cooling when the surfacetemperature of the hot-rolled plate obtained through the processdescribed above is Ar3˜20° C.˜Ar3+50° C.

When the start temperature of primary cooing exceeds Ar3+50° C., phasetransformation into ferrite is not sufficiently made at the surfaceportion during primary cooling, so a hardness reduction effect at thesurface portion cannot be achieved. However, when the start temperatureof primary cooing is less than Ar3−20° C., ferrite transformation isexcessive generated even to the center portion, which is a factor thatreduces strength of steel.

Further, it is preferable to perform the primary cooling at a coolingrate of 5˜40° C./s such that the surface temperature of the hot-rolledplate becomes Ar1−50° C.˜Ar3−50° C.

That is, when the end temperature of the primary cooling exceeds Ar3−50°C., the fracture of phase transformation into ferrite at the surfaceportion of the primarily cooled hot-rolled plate is low, so the hardnessreduction effect at the surface portion cannot be sufficiently achieved.However, when the temperature is less than Ar1−50° C., ferrite phasetransformation excessively occurs even to the center portion, so it isdifficult to secure strength at a target level.

Further, when the cooling rate in the primary cooling is excessively lowless than 5° C./s, it is difficult to primary cooling end temperaturedescribed above, but when the cooling rate exceeds 40° C./s, thefracture of phase transformation into acicular ferrite, so a softstructure cannot be formed at the surface portion. Accordingly, in theprimary cooling, for the temperature at the surface portion, it ispossible to control the average cooling rate at 5˜40° C./s, and morepreferably, 17˜40° C./s.

When the primary cooling is ended, the temperature at the center portionof the hot-rolled plate may be controlled at Ar3˜30° C.˜Ar3+30° C. Thatis, when the temperature at the center portion of the hot-rolled plateexceeds Ar3+30° C. at the end of the primary cooling, the temperature ofthe surface portion cooled within a specific temperature range isincreased, so the fracture of ferrite phase transformation of thesurface portion is decreased. Accordingly, the temperature at the centerportion of the hot-rolled plate may be controlled preferably at 730˜810°C. at the end of the primary cooling.

However, when the temperature at the center portion of the hot-rolledplate is less than Ar3−30° C., the temperature of the center portion ofthe hot-rolled plate is excessively decreases and the temperature atwhich the surface portion can be recuperated in the following coolingdecreases, so a tempering effect cannot be achieved, which decreases thehardness reduction effect at the surface portion.

Air Cooling

It is preferable to air-cool the hot-rolled plate that has undergoneprimary cooling under the conditions described above, and an effect ofrecuperation of the surface portion can be obtained by the centerportion that is a relatively high temperature through the air-coolingprocess.

It is preferable to end the air cooling when the temperature of thesurface portion of the hot-rolled plate becomes the rang Ar3−50°C.˜Ar3−10° C.

When the temperature of the surface portion of the hot-rolled plate islower than Ar3−50° C. after the air cooling is finished, not only thetime for formation of air-cooled ferrite is insufficient, but thetempering effect by recuperation of the surface portion is insufficient,which is disadvantageous in hardness reduction of the surface portion.However, when the temperature of the surface portion of the hot-rolledplate exceeds than Ar3−50° C. after the air cooling is finished, coolingtime excessively increases and ferrite phase transformation occurs atthe center portion, so it is difficult to secure strength at a targetlevel.

Secondary Cooling

It is preferable to perform secondary cooling immediately after the aircooling is finished within the temperature range described above (basedon the temperature of the surface portion), and the temperature of thesurface portion at the end of air cooling is the same as the start pointin secondary cooling.

Meanwhile, it is preferable that the secondary cooling is performed at acooling rate of 50˜500° C./s such that the temperature of the surfaceportion becomes 300˜600° C.

That is, when the end temperature of the secondary cooling is less than300° C., the fracture on MA increases, which has a bad influence onsecurity of low-temperature toughness and suppression of hydrogenembrittlement. However, when the end temperature of the secondarycooling exceeds 600° C., phase transformation is not completed in thecenter portion, so it is difficult to secure strength.

Further, the cooling rate is less than 50° C./s in secondary coolingwithin the temperature range described above, the grains at the centerportion are coarsened, so it is difficult to secure strength at a targetlevel. However, when the cooling rate exceeds 500° C./s, the fracture ofa phase vulnerable to low-temperature toughness such as upper bainite isincreased due to a microstructure at the center portion, so hydrogeninduced cracking resistance is deteriorated, which is disadvantageous.Accordingly, in the secondary cooling, for the temperature at thesurface portion, it is possible to control the average cooling rate at50˜500° C./s, and more preferably, 245˜500° C./s.

Meanwhile, according to an aspect of the present disclosure, a steelmaterial manufactured through the sequence of processes may havethickness of 5˜50 mm.

MODE FOR INVENTION

Hereafter, the present disclosure is described in more detail throughembodiments. However, it should be noted that the following embodimentsare provided only to describe the present disclosure in more detailthrough exemplification rather than limiting the right range of thepresent disclosure. This is because the right range of the presentdisclosure is determined the matters described in claims and mattersreasonably inferred from the matters.

Embodiment

Steel slabs having the alloy composition and properties shown in thefollowing Tables 1 and 2 were prepared. In this case, the content of thefollowing ally composition is described in percent by weight and thebalance includes Fe and other unavoidable impurities. Steel materialswas manufactured by heating, hot-rolling, and cooling the prepared steelslabs, respectively, under the conditions shown in Tables 3 and 4.

The invention steel and comparative steel described in Tables 1 and 2were manufactured through the same processes except for following themanufacturing conditions described in Tables 3 and 4.

In detail, the steel materials of the invention steel and comparativesteel were obtained by heating slabs having the composition described inthe following Table 1 under the conditions described in Table 3,performing rough rolling under common conditions, performing finishinghot-rolling under the conditions described in Table 3, and thenperforming water cooling after maintaining resting for a predeterminedtime. Cooling described Table 4 was controlled by performingintermediate air cooling and then secondary cooling after primarycooling.

TABLE 1 C Si Mn P S Al N Ni Cr Mo Nb Ti V Ca IS* 1 0.043 0.25 1.32 0.0060.0007 0.024 0.003 0.21 0.002 0.12 0.043 0.012 0.02 0.0018 IS 2 0.0440.24 1.31 0.008 0.0005 0.023 0.004 0.18 0.007 0.14 0.041 0.013 0 0.0016IS 3 0.043 0.23 1.33 0.009 0.0008 0.025 0.004 0.15 0.02 0.12 0.046 0.0110 0.0011 CS* 1 0.11 0.25 1.44 0.008 0.0008 0.031 0.005 0.21 0.03 0.060.05 0.011 0.02 0.0015 CS 2 0.036 0.24 1.55 0.008 0.0008 0.029 0.006 00.21 0 0.035 0.012 0.02 0.0011 CS 3 0.037 0.22 1.22 0.006 0.001 0.0380.004 0.16 0.19 0 0.044 0.013 0 0.0004 CS 4 0.043 0.25 1.32 0.006 0.00070.024 0.003 0.21 0.002 0.12 0.043 0.012 0.02 0.0018 CS 5 0.043 0.25 1.320.006 0.0007 0.024 0.003 0.21 0.002 0.12 0.043 0.012 0.02 0.0018 CS 60.043 0.25 1.32 0.006 0.0007 0.024 0.003 0.21 0.002 0.12 0.043 0.0120.02 0.0018 CS 7 0.043 0.25 1.32 0.006 0.0007 0.024 0.003 0.21 0.0020.12 0.043 0.012 0.02 0.0018 CS 8 0.043 0.25 1.32 0.006 0.0007 0.0240.003 0.21 0.002 0.12 0.043 0.012 0.02 0.0018 CS 9 0.043 0.25 1.32 0.0060.0007 0.024 0.003 0.21 0.002 0.12 0.043 0.012 0.02 0.0018 IS*Inventivesteel CS*Comparative steel

TABLE 2 Ca/S Ar3 (° C.) Ar1 (° C.) IS 1 2.6 778 717 IS 2 3.2 775 718 IS3 1.4 776 719 CS 1 1.9 752 715 CS 2 1.4 780 726 CS 3 0.4 797 722 CS 42.6 777 717 CS 5 2.6 777 717 CS 6 2.6 777 717 CS 7 2.6 777 717 CS 8 2.6777 717 CS 9 2.6 777 717 Ar3 = 910 − 310 × C − 80 × Mn − 20 × Cu − 15 ×Cr − 55 × Ni − 80 × Mo + 0.35 × (thickness [mm] − 8) Ar1 = 742 − 7.1 × C− 14.1 × Mn + 16.3 × Si + 11.5 × Cr − 49.7 × Ni

TABLE 3 Hot rolling Maintaining Slab heating Accumulated time afterHeating Heating Finishing reduction finishing Thickness temperature timetemperature ratio rolling [mm] [° C.] [hr] [° C.] [%] [sec] IS 1 30.51166 4.3 893 80 72 IS 2 21.5 1158 4 918 77 135 IS 3 19.5 1145 3.9 905 77188 CS 1 30.5 1129 4.3 850 75 122 CS 2 30.5 1127 4.2 875 75 135 CS 330.5 1133 3.9 895 77 138 CS 4 30.5 1131 4.5 888 80 180 CS 5 30.5 11323.7 895 77 185 CS 6 30.5 1145 4.3 879 75 194 CS 7 30.5 1155 3.6 834 75171 CS 8 30.5 1050 3.1 870 75 139 CS 9 30.5 1145 4.4 865 75 11

TABLE 4 Primary Primary Primary Temperature Secondary Secondary Primarycooling end cooling end cooling of surface cooling end cooling 2-cooling temperature temperature rate of portion after temperature rateof step start of surface of center surface intermediate of surfacesurface cooling temperature portion portion portion air cooling portionportion or not [° C.] [° C.] [° C.] [° C./s] [° C.] [° C.] [° C./s] IS 10 825 710 802 22 750 466 345 IS 2 0 815 699 799 13 754 489 321 IS 3 0822 703 799 17 748 443 245 CS 1 X 815 492 495 245 — — — CS 2 X 780 488494 255 — — — CS 3 X 823 503 495 261 — — — CS 4 X 823 465 483 359 — — —CS 5 0 823 611 732 25 642 455 324 CS 6 0 820 718 789 123 760 444 359 CS7 0 743 616 702 21 688 466 321 CS 8 0 818 698 794 16 754 455 324 CS 9 0820 700 795 15 755 454 333

Yield strength, Vickers hardness in the surface portion, sulfide stresscorrosion cracking resistance, a microstructure of each of the steelmaterials manufactured through the manufacturing process described abovewere observed, and the result was shown in the following Table 5.

In this case, yield strength is 0.5% under-load yield strength, API-5Lspecimens were taken in a direction perpendicular to the rollingdirection as the tension samples, and the tests were performed.

Hardness of the steel materials was measured on thickness cross-sectionsunder 1 kgf load using a Vickers hardness tester, and hardness of thesurface portions were measured from the surface portion to positions at100 μm and were shown in the following Table 5.

Meanwhile, microstructures were measured using an optical microscope andthe kinds of phases were observed using an image analyzer.

A 4 Point Bent Beam Test was performed for characteristic analysis ofsulfide stress corrosion cracking (SSC) under NACE standard test method(TM-0177), and whether cracking occurred was estimated by adding 90% ofyield strength of each steel plate to a strong acid Sol. A solution andthen exposing the solution in an H₂S environment of bar for 720 hours.

TABLE 5 Hardness Sulfide stress Structure Structure of surface Yieldcorrosion of surface of center portion strength cracking portion portion[Hv] [MPa] [SSC] Invention IS 1 F + P AF 172 478 Not steel generated IS2 F + P AF 183 489 Not generated IS 3 F + P AF 178 490 Not generatedComparative CS 1 UB AF + IJB 284 534 Generated steel CS 2 UB AF + UB 275545 Generated CS 3 AF AF 224 483 Generated CS 4 AF AF 228 478 GeneratedCS 5 F + P AF + F + P 175 421 Not generated CS 6 AF AF 224 475 GeneratedCS 7 F + P F + P 175 411 Generated CS 8 F + AF AF + F 202 452 GeneratedCS 9 F + AF AF + F 205 475 Generated F: Ferrite, P: Pearlite, AF:Acicular Ferrite, UB: Upper Bainite

In Tables 1 to 5, the invention steels satisfied both the compositionand manufacturing conditions of the present disclosure and thecomparative steels did not satisfy any one or more the composition andmanufacturing conditions of the present disclosure.

In detail, the comparative steels 1 to 4 did not satisfy both thecomposition and manufacturing conditions of the present disclosure, andparticularly, the 2-step cooling method proposed in the presentdisclosure was not applied in cooling.

Meanwhile, the comparative steels 4 to 9 used steel slabs having thesame composition as the invention steel 1 of the present disclosure anddid not satisfy the manufacturing conditions of the present disclosure.That is, the 2-step cooling method proposed in the present disclosurewas not applied to the comparative steel 4, and, in the comparativesteel 5, a primary cooling end temperature of the surface portion andthe temperature of the surface portion after intermediate air coolingwere out of the range proposed in the present disclosure.

Further, the primary cooling rate of the surface portion was out of therange proposed in the present disclosure in the comparative steel 6, thefinishing temperature of hot rolling was out of the lower limit rangeproposed in the present disclosure in comparative steel 7, and thefinishing temperature of hot rolling was decreased, so all of theprimary cooling start temperature, the primary cooling end temperaturesof the surface portion and the center portion, and the temperature ofthe surface portion after intermediate air cooling were all out of theranges proposed in the present disclosure.

The heating temperature of the slab was out of the lower limit rangeproposed in the present disclosure in the comparative steel 8, and themaintaining time after finishing hot rolling was out of the lower limitrange proposed in the present disclosure in the comparative steel 9.

The 2-step cooling proposed in the present disclosure was not applied tothe comparative steels 1 to 4, so a ferrite structure of a complexstructure of ferrite and pearlite proposed in the present disclosure wasnot formed in the microstructures of the surface portions. Accordingly,the hardness in the surface portions exceeded 200 Hv in the comparativesteels 1 to 4, so sulfide stress corrosion cracking was generated due tohigh hardness in the surface portions.

2-step cooling proposed in the present disclosure was not applied to thecomparative steel 1, but the primary cooling end temperature of thecenter portion and the temperature of the surface portion afterintermediate air cooling were low, so ferrite transformation wasgenerated before secondary cooling. In the comparative steel 5, sulfidestress corrosion cracking was the generated, but the yield strength didnot satisfy 450 MPa or more that is the range set in the presentdisclosure.

In the comparative steel 6, the primary cooling rate exceeded the upperlimit proposed in the present disclosure and ferrite was not formed atthe surface portion, so sulfide stress corrosion cracking was generated.

In the comparative steel 7, the finishing temperature of hot rolling didnot satisfy the lower limit proposed in the present disclosure, in whichthe cooling temperature after hot rolling also did not satisfy the rangeproposed in the present disclosure, so ferrite transformation wasgenerated even to the center portion, and accordingly, the yieldstrength was insufficient.

The heating temperature of the slab was out of the range proposed in thepresent disclosure in the comparative steel 8 and the maintaining timeafter hot rolling was out of the range proposed in the presentdisclosure in the comparative steel 9. In the comparative steels 8 and9, since ferrite transformation was insufficient at the surfaceportions, so a complex structure of ferrite and acicular ferrite wasformed, whereby the surface portion hardness reduction effect was notsufficiently achieved and sulfide stress corrosion cracking wasgenerated.

As described above, in the invention steels 1 to 3 that satisfy both thealloy composition and the manufacturing conditions proposed in thepresent disclosure, the hardness in the surface portions is 200 Hv orless, so the hardness in the surface portion is remarkably low and yieldstrength of 450 MPa or more could be secured. Further, it could be seenthat resistance against sulfide stress corrosion cracking was alsoexcellent.

However, in the comparative steels 1˜9 that did not satisfy the alloycomposition of the present disclosure or did not satisfy themanufacturing conditions of the present disclosure, the hardness in thesurface portions of the steel materials was not sufficiently low, sosulfide stress corrosion cracking was generated or yield strength of 450MPa or more could not be secured.

Meanwhile, microstructure pictures at the surface portions and thehardness values at the surface portions measured by an opticalmicroscope for the invention steel 2 and the comparative steel 3 of theabove test examples were shown in FIG. 1 . In detail, in FIG. 2 , theleft pictures show hardness measured from a surface to a position at 100μm using a Vickers hardness tester, and the right pictures show hardnessmeasured from a surface to a position at 500 μm.

As can be seen from FIG. 1 , it can be seen that the steel material ofthe present disclosure has hardness of 200 Hv at the surface portion,but the hardness in the surface portion exceeds 200 Hv in thecomparative steel 3 to which 2-step cooling proposed in the presentdisclosure was not applied.

1. A steel material comprising, by weight %, carbon (C): 0.02˜0.06%,silicon (Si): 0.1˜0.5%, manganese (Mn): 0.8˜1.8%, chrome (Cr): less than0.05%, phosphorous (P): 0.03% or less, sulfur (S) 0.003% or less,aluminum (Al): 0.06% or less, nitrogen (N): 0.01% or less, niobium (Nb):0.005˜0.08%, titanium (Ti): 0.005˜0.05%, calcium (Ca): 0.0005˜0.005%;one or more of nickel (Ni): 0.05˜0.3%, molybdenum (Mo): 0.02˜0.2%, andvanadium (V): 0.005˜0.1%, and a balance of Fe and unavoidableimpurities, wherein the Ca and the S satisfy the following Equation 1,the steel material has a microstructure of a surface portion composed offerrite or a complex structure of ferrite and pearlite, and amicrostructure of the center portion is composed of acicular ferrite,0.5≤Ca/S≤5.0  [Equation 1] where each element represents the content ofeach element by weight %.
 2. The steel material of claim 1, whereinVickers hardness in the surface portion is 200 Hv or less.
 3. The steelmaterial of claim 1, wherein the steel material has yield strength of450 MPa or more.
 4. A method of manufacturing a steel material, themethod comprising: heating a steel slab, which includes, by weight %,carbon (C): 0.02˜0.06%, silicon (Si): 0.1˜0.5%, manganese (Mn) 0.8˜1.8%,chrome (Cr): less than 0.05%, phosphorous (P): 0.03% or less, sulfur(S): 0.003% or less, aluminum (Al) 0.06% or less, nitrogen (N): 0.01% orless, niobium (Nb) 0.005˜0.08%, titanium (Ti): 0.005˜0.05%, calcium(Ca): 0.0005˜0.005%; one or more of nickel (Ni): 0.05˜0.3%, molybdenum(Mo): 0.02˜0.2%, and vanadium (V): 0.005˜0.1%, and a balance of Fe andunavoidable impurities and in which the Ca and the S satisfy thefollowing Equation 1, at a temperature range of 1100˜1300° C. for 2hours or more; obtaining a hot-rolled plate by hot-rolling the heatedsteel slab; and cooling the hot-rolled plate after hot rolling, whereinthe cooling includes primary cooling, air cooling, and secondarycooling, and the primary cooling is performed at a cooling rate of 5˜40°C./s such that a temperature of a surface portion of the hot-rolledplate becomes Ar1−50° C.˜Ar3−50° C. and the secondary cooling isperformed at a cooling rate of 50˜500° C./s such that the temperature ofthe surface portion of the hot-rolled plate becomes 300˜600° C.0.5≤Ca/S≤5.0  [Equation 1] where each element represents the content ofeach element by weight %.
 5. The method of claim 4, wherein the hotrolling is performed at an accumulated reduction ratio of 50% or more ina temperature range of Ar3+80° C.˜Ar3+200° C.
 6. The method of claim 4,further comprising maintaining for 30 second or more before coolingafter the hot rolling.
 7. The method of claim 4, wherein the primarycooling is started when the temperature of the surface of the hot-rolledplate is Ar3−20° C.˜Ar3+50° C.
 8. The method of claim 4, wherein atemperature of a center portion of the hot-rolled plate is Ar3−30°C.˜Ar3+30° C. after the primary cooling is finished.
 9. The method ofclaim 4, wherein the temperature of the surface portion of thehot-rolled plate is Ar3−10° C.˜Ar3−50° C. after the air cooling isfinished.